The nitrogen oxide selective catalytic reduction (SCR) system for engine exhaust purification and the diesel particulate filter (DPF) system are not only of great importance to diesel engines, but also have application value for gasoline direct injection engines. Both relate to the liquid injection and measurement.
In a nitrogen oxide selective catalytic reduction (SCR) system, it is necessary to quantitatively inject 32.5% urea solution into the engine exhaust system. At a high temperature, the urea solution is decomposed into ammonia gas, which reacts, in the presence of a SCR catalyst, with nitrogen oxide in the exhaust gas to produce nitrogen gas and water, thereby reducing emission.
In a diesel particulate filter (DPF) system, a ceramic or metal carrier is used to filter the soot to prevent it from being discharged into the air. However, as the soot particulates accumulate in the filtration carrier, the engine exhaust back pressure rises continuously. At this time, it is necessary to inject fuel into the exhaust system to increase the discharge temperature and burn up the accumulated soot, thereby achieving regeneration of the diesel particulate filter (DPF).
The above processes have strict requirements for the precision of the liquid injection amount. For SCR system, injection of too little NOX reducing agent (i.e. urea solution) will not lead to reduction of some nitrogen oxides, and injection of too much reducing agent will cause secondary pollution as well as waste. For DPF regeneration system, insufficient fuel injection may cause inability to complete regeneration, and excessive fuel injection may lead to a too high temperature, and burn up the DPF filtration carrier. Therefore, for the above applications, accurate injection and measurement is very important.
In prior arts, there are two primary technical routes of injections and measurements. One route is as follows: a liquid feed pump system provides a constant pressure, and the injection amount is measured on the basis of the opening duration of an electromagnetic nozzle, such as the common rail system of gasoline and diesel, and the DENOXTRONIC 2.2 sold by BOSCH and other urea injection systems. The other route is by adopting a piston pump metering device that is driven by a solenoid device, and the injection amount of each pulse is measured on the basis of the displacement of a piston.
For the latter, U.S. Patent No.: US20150082775A discloses a metering device that is a solenoid-driven piston pump, wherein the piston sleeve is fixed with the solenoid, and the piston is fixed on an armature and reciprocates along a fixed stroke in a T-shaped space. The injection amount of a single pulse is determined by the motion stroke of the piston. The disadvantage of such a metering device is that the piston connects with the armature end to end, the axial dimension is relatively long, and the moving mass is relatively large. Therefore, if friction pairs completely rely on piston-sleeve coordination, then the center of gravity of the moving parts will be outside the support friction pairs, and the movement inertia will affect the durability. If a slide support (i.e. the MAGNET SLEEVE 67) of the piston or of the armature is arranged on the other end of the spring opposite to the sleeve, then it is necessary to control the co-axiality of the sleeve and the slide support. However, it is difficult to make such structure particularly to arrange an extra snap ring (snap ring 68) between the slide support of the piston and the spring, because it is complicated to install a snap ring. In addition, as the mass and friction force of the moving part are relatively large, the frequency of reciprocating motion of the piston will be restricted.